The invention relates to an exhaust gas after-treatment unit for an internal combustion engine, in particular of a motor vehicle.
Exhaust gas after-treatment units for internal combustion engines, especially for motor vehicles, have long been known from the general prior art and particularly from serial vehicle manufacturing.
The internal combustion engine has at least one combustion chamber, in particular in the form of a cylinder, into which fuel especially liquid fuel, and air are fed during operation of the internal combustion engine with the throttle open. This creates in the combustion chamber a fuel-air mixture, which is also called the mixture and is burned. This results in exhaust gas from the internal combustion engine, wherein the exhaust gas can flow out from at least one internal combustion engine outlet and, therefore, out of the internal combustion engine itself.
By means of an exhaust pipe, for example, the exhaust gas is directed towards the exhaust gas after-treatment unit so that the internal combustion engine exhaust gas can be after-treated using the exhaust gas after-treatment unit. To this end, the exhaust gas after-treatment unit comprises at least one SCR catalytic converter through which the internal combustion engine exhaust gas can flow and by means of which a selective catalytic reduction (SCR) is brought about and supported. This means that the SCR catalytic converter catalyzes the SCR reaction. The nitrogen oxide (NOx) contained within the exhaust gas is reduced by this selective catalytic reduction, meaning that it is at least partially removed from the exhaust gas. This removal of the nitrogen oxide from the exhaust gas is also known as denitrification. In the course of the SCR, the nitrogen oxide contained within the exhaust gas reacts in particular with elements of a reduction agent which is introduced to the exhaust gas or with elements which form from the reduction agent to become nitrogen and water, in particular, the reduction agent is an aqueous urea solution. Ammonia (NH3), which acts to reduce nitrogen oxide in the SCR, is formed from this aqueous urea solution.
Moreover, particularly exhaust gas after-treatment units for internal combustion engines that are configured as diesel engines comprise at least one particle filter, through which the exhaust gas can flow and which is arranged upstream of the SCR catalytic converter—in the flow direction of the exhaust gas through the exhaust gas after-treatment unit—to retain soot particles from the exhaust gas. The exhaust gas is filtered by means of the particle filter so that at least some of the soot particles contained in the exhaust gas are filtered out of the exhaust gas by means of the particle filter. If the internal combustion engine is configured as a diesel engine, then the particle filter is usually also described as a diesel particle filter (DPF).
Additionally, modern exhaust gas after-treatment units, especially those intended for diesel engines, usually have an oxidation catalytic converter (DOC) upstream of the particle filter in the flow direction of the exhaust gas through the exhaust gas after-treatment unit.
The objective of the present invention is to further develop an exhaust gas after-treatment unit of the aforementioned type which allows for the implementation of an operation that is especially favorable in terms of exhaust emissions.
In order to further develop an exhaust gas after-treatment unit in such a way that it is possible to implement an operation that is especially favorable in terms of exhaust emissions, the invention provides that the exhaust gas after-treatment unit has a combination catalytic converter, through which the exhaust gas can flow and which is arranged upstream of the particle filter. The combination catalytic converter comprises a first catalytic converter part, which is configured as an SCR catalytic converter or an SCR catalytic converter part. This means that the first catalytic converter part brings about and/or supports a selective catalytic reduction (SCR), wherein nitrogen oxides (NOx) contained in the exhaust gas are reduced, i.e., at least partially removed from the exhaust gas, within the framework of the SCR reaction, in the course of the SCR reaction, the nitrogen oxide contained in the exhaust gas reacts in particular with elements of a reduction agent which is introduced to the exhaust gas or with elements which form from the reduction agent to become nitrogen and water. Here the first catalytic converter part catalyzes the SCR reaction and thus has an SCR effect, and so, with the aid of the first catalytic converter part, nitrogen monoxide (NO) and/or nitrogen dioxide (NO2) in particular can be converted into nitrogen (N2).
The combination catalytic converter further comprises a second catalytic converter part that is arranged downstream of the first catalytic converter part in the direction of flow of the exhaust gas through the exhaust gas after-treatment unit. This means that the exhaust gas flowing through the exhaust gas after-treatment unit initially flows through the first catalytic converter part and subsequently through the second catalytic converter part. The second catalytic converter part is configured as an ammonia slip catalytic converter and has a layer of noble metal with a first noble metal content. In the ammonia slip catalytic converter, ammonia slip from the reduction agent is oxidized into nitrogen and water. An ASC effect in this instance is understood to be a catalytic effect on the oxidation of ammonia (NH3) by the ammonia slip catalytic converter.
The combination catalytic converter further comprises a third catalytic converter part, which is arranged downstream of the second catalytic converter part. This means that the exhaust gas flowing through the exhaust gas after-treatment unit initially flows through the first catalytic converter part, then through the second catalytic converter part and subsequently through the third catalytic converter part, and so the exhaust gas flows through the first, second and third catalytic converter parts in succession. The third catalytic converter part is configured as an oxidation catalytic converter and has a layer of noble metal with a second noble metal content. The oxidation catalytic converter, and thus the third catalytic converter part, perform the task of oxidizing any carbon monoxide (CO) contained in the exhaust gas and any hydrocarbons (HC) contained in the exhaust gas. Therefore, the third catalytic converter part catalyzes, i.e., brings about or supports, the oxidation of uncombusted hydrocarbons and carbon monoxide, and so the third catalytic converter part has an OC effect, in particular a DOC effect. Furthermore, the combination catalytic converter has an SCR layer, especially an upper layer, disposed on the layers of noble metals of the second and third catalytic converter parts, the layer being configured as a copper-zeolite layer (Cu—Z layer), for example, and extending over the entire length L of the second and third catalytic converter parts.
The SCR layer is a fourth component of the combination catalytic converter, for example, wherein the SCR layer is arranged and/or applied to the noble metal layers that extend into deeper wall layers, and so the exhaust gas escaping from the first catalytic converter part and flowing into the second catalytic converter part initially contacts the SCR layer and then diffuses into the deeper noble metal layers of the second catalytic converter part. The SCR layer should be understood such that the SCR layer has an SCR effect, and reduction reactions of nitrogen oxide with ammonia into nitrogen and water vapor are catalyzed in the context of this effect. In the second and third catalytic converter parts of the combination catalytic converter according to the invention, the SCR layer is provided to degrade NH3 slip from the first catalytic converter part by SCR reactions. The SCR layer is thus advantageously disposed directly on the noble metal layers and thereby contacts the respective noble metal layers. It is particularly advantageous for the combination catalytic converter to be configured such that the layers of noble metals and the SCR layer of the second and third catalytic converter parts are applied to the same main body elements of the catalytic converter, and the second and third catalytic converter parts directly adjoin and, if possible, contact each other in the flow direction of the exhaust gas, as a result of which the second and third catalytic converter parts advantageously can be configured to be especially compact and can be produced especially cost-effectively.
Using the exhaust gas after-treatment unit according to the invention, it is possible to prevent excessive nitrogen oxide emissions (NOx emissions), particularly after a start, especially after a cold start, of the internal combustion engine as well as after operating the internal combustion engine in a low-load range. The invention is particularly based on the finding that high nitrogen oxide emissions can normally be generated, particularly after a start, especially after a cold start, of the internal combustion engine as well as following a motor vehicle operation in the low-load range, particularly following an idle operation, including coasting mode, during which the internal combustion engine is in its idle operation, as well as after traffic light waiting periods, during which the internal combustion engine is running and is in its idle operation, since the catalytic converters and filters for the exhaust gas after-treatment unit cool down in these motor vehicle operation modes and are so cold following these motor vehicle operation modes that the catalytic converters and filters must first be brought up to working temperature during the subsequent start-up processes or acceleration processes, during which very high exhaust emissions are produced. The exhaust gas after-treatment unit according to the invention is advantageously especially well-suited for diesel vehicles and especially for diesel trucks, whose emissions, in particular nitrogen oxide emissions, can be kept particularly low by means of the exhaust gas after-treatment unit according to the invention.
More particularly, the invention is based on the finding that future emissions requirements for internal combustion engines, especially diesel engines, will increasingly be aimed at considering various secondary emissions, such as NO2 and N2O, as well as the working capacity of the exhaust gas after-treatment unit in real driving conditions. A rapid warm-up of the SCR catalytic converter, especially after a cold start, and thus high efficiency of the SCR catalytic converter are prevented in conventional exhaust gas after-treatment units, which comprise an oxidation catalytic converter, a particle filter arranged downstream of the oxidation catalytic converter, an SCR catalytic converter downstream of the particle filter and an ASC (ammonia slip catalytic converter) downstream of the SCR catalytic converter. Advantageously, the first catalytic converter part in the combination catalytic converter of the exhaust gas after-treatment unit according to the invention is configured as an SCR catalytic converter and is the first exhaust gas after-treatment unit in the flow direction after the escape of the exhaust gas from the internal combustion engine, and so the exhaust gas temperatures in the first catalytic converter part of the exhaust gas after-treatment unit according to the invention are comparatively high. As a result, the first catalytic converter part, which is configured as an SCR catalytic converter, heats up relatively quickly after a start or low-load operation by the internal combustion engine, and so it is also possible to achieve higher denitrification efficiency in the catalytic converter part configured as an SCR catalytic converter relatively fast after a start or low-load operation by the internal combustion engine. Hereafter, the first catalytic converter part of the exhaust gas after-treatment unit, which is configured as an SCR catalytic converter, will be referred to as the first SCR catalytic converter.
The cooling of the catalytic converters and filters after a cold start or low-load operations by the internal combustion engine is more pronounced with correspondingly higher exhaust gas emissions in motor vehicles in the form of commercial vehicles or heavy-goods vehicles than with passenger cars, since there is a comparatively larger interval with an associatively large distance between the internal combustion engine and an exhaust gas after-treatment unit in commercial vehicles or heavy-goods vehicles than in passenger cars, whereby higher thermal losses occur in commercial vehicles or heavy-goods vehicles than in passenger cars. In a conventional exhaust gas after-treatment unit, the introduction of a reduction agent into the exhaust gas is discontinued under the above-mentioned operating conditions; in other words, in and for a heating phase following a start, particularly a cold start, and also for a heating phase following a low-load operation, since the exhaust gas has a very low temperature under these operating conditions. The introduction of reduction agent is discontinued here so that the reduction agent does not crystallize. The introduction of the reduction agent is normally activated or implemented only when an SCR catalytic converter, in which the reaction agent should be implemented, has a temperature higher than 180 degrees Celsius. Discontinuing the introduction of the reduction agent results in high nitrogen oxide emissions during the specified operating conditions no appropriate countermeasures are undertaken.
Extremely good cold start behavior and advantageous behavior in real driving conditions can be realized by the use of the exhaust gas after-treatment unit according to the invention. Secondary NO2 emissions are kept low, particularly during urban operation, especially from NO2 proportions less than or equal to 50 percent. It has also been shown to be especially beneficial for the first SCR catalytic converter to be smaller than the vanadium-based SCR catalytic converter. Advantageously, in vanadium-based SCR catalytic converters, a comparatively low ammonia fill level is necessary for good denitrification efficiency.
In an advantageous embodiment of the invention, the layers of noble metal are formed from platinum or mixtures of platinum and palladium, and the second noble metal content is higher than the first noble metal content. Noble metal layers consisting of platinum and palladium exhibit high NO2-forming activity and a high catalytic effect for HC oxidation. Furthermore, this embodiment of the invention is based on the following knowledge: The higher the noble metal content of a catalytic converter, the higher the reaction rate of an NH3 formed from the reduction agent into nitrous oxide (N2O) compared to a reaction rate of NH3 into N2. Owing to the lower content of noble metal in the noble metal layer of the second catalytic converter part, an NH3 slip from the first SCR catalytic converter is advantageously substantially oxidized into N2 instead of N2O in the second catalytic converter part, and so no ammonia or only a very small amount of ammonia reaches the third catalytic converter part, which is configured as an oxidation component or oxidation catalytic converter. Due to the higher noble metal content of the noble metal layer provided in the oxidation catalytic converter, incoming ammonia is repeatedly converted to N2O, which is prevented by the second catalytic converter part. With this configuration of the invention, the emissions of climate-affecting N2O can be kept low, while it is still possible to provide a hot first SCR catalytic converter near the internal combustion engine, with the associated necessary addition of NH3 via a reduction agent upstream of an oxidation catalytic converter in the exhaust gas after-treatment unit.
In one embodiment of the invention, the noble metal layer of the second catalytic converter part has a higher platinum content than the noble metal layer of the third catalytic converter part in a mixture of platinum and palladium, where the platinum content is at least 80 percent of the entire mixture. The noble metal layer of the second catalytic converter part can also be composed exclusively of platinum.
In one embodiment of the invention, the noble metal layer of the third catalytic converter part has a platinum content of at least 50 percent in a total mixture of platinum and palladium.
Moreover, it has proven especially advantageous when the first noble metal content is in a range from approximately 1/28316.8 grams per cubic centimeter to approximately 5/28316 grams per cubic centimeter, inclusive. This means that the first noble metal content preferably lies in a range from 1 gram of noble metal per cubic foot to approximately 5 grams of noble metal per cubic foot, inclusive, wherein one cubic foot corresponds at least substantially to 28316.8 cubic centimeters. With the first noble metal content of the second catalytic converter part according to this embodiment of the invention, an NH3 slip from the first SCR catalytic converter can advantageously be substantially oxidized into N2 instead of N2O, and so no ammonia or only a very small amount of ammonia reaches the third catalytic converter part, which is configured as an oxidation catalytic converter.
Finally, it has proven especially advantageous when the second noble metal content is in a range from approximately 5/28316.8 grams per cubic centimeter to approximately 20/28316.8 grams per cubic centimeter, inclusive. With the second noble metal content of the third catalytic converter part according to this embodiment of the invention, it is advantageously possible to achieve a high oxidation rate of HC and a high oxidation rate of NO3 to NO2 in the third catalytic converter part. A high NO2 content at the outlet of the combination catalytic converter and thus before the intake of the exhaust gas into the particle filter advantageously increases a passive regeneration of the particle filter with NO2.
In a further embodiment of the invention, the exhaust gas after-treatment unit is provided with a first metering device, by means of which a reduction agent, in particular an aqueous urea solution, can be introduced into the exhaust gas in at least one location upstream of the combination catalytic converter and thus upstream of the first SCR catalytic converter in order to denitrify the exhaust gas. In this way, nitrogen can be removed from the exhaust gas in an especially effective way so that the emissions, particularly nitrogen oxide emissions, can be kept especially low.
In order to keep nitrogen oxide emissions especially low, a second metering device is provided in a further embodiment of the invention, by means of which a reduction agent, in particular an aqueous urea solution, can be introduced into the exhaust gas in at least one location downstream of the first SCR catalytic converter and thus downstream of the second catalytic converter in order to denitrify the exhaust gas. The second metering device is employed advantageously to provide the reduction agent in the exhaust gas before entry into the second catalytic converter, which is configured as an SCR catalytic converter, since the NH3 slip is oxidized in the second catalytic converter part for the function of the third catalytic converter part, which acts as an oxidation catalytic converter, and so essentially no more NH3 is present in the exhaust gas downstream of the second catalytic converter part of the combination catalytic converter and thus also in the second catalytic converter, which is arranged downstream of the combination catalytic converter. Hereafter, the second catalytic converter part of the exhaust gas after-treatment unit, which is configured as an SCR catalytic converter, will be referred to as the second SCR catalytic converter.
Owing to the use of the combination catalytic converter and the arrangement of the metering devices according to this embodiment of the invention, the NO2-based passive regeneration of the particle filter can take place over particularly long periods of time or almost all the time, since during the NO2-based regeneration of the particle filter, during which the metering of a reduction agent by the first metering device is discontinued, the second metering device can be enabled or activated, and, with the second catalytic converter, a nitrogen oxide reduction can be carried out with the reduction agent introduced by the second metering device. Therefore, advantageously, there is no time limit on the reduction of nitrogen oxide during an NO2-based regeneration of the particle filter in this embodiment of the invention, which would be dictated by a storage capacity for NH3 of the second SCR catalytic converter.
Furthermore, it has been shown to be especially advantageous when the second location at which the reduction agent can be introduced by means of the second metering device is disposed downstream of the particle filter.
In a further embodiment of the invention, the particle filter is provided with a catalyzing coating that is free of heavy metals and precious metals and that oxidizes the soot particles retained by the particle filter. The heavy metal- and precious metal-free particle filter coating in the exhaust gas after-treatment unit according to this embodiment of the invention advantageously contains no environmentally damaging heavy metals and no other toxic or environmentally damaging materials.
In a further embodiment of the invention, the heavy metal- and precious metal-free particle filter coating contains alkaline and/or alkaline-earth compounds. Especially advantageous here is that the heavy metal- and precious metal-free particle filter coating comprises silicates containing alkaline metals. Particle filters with this type of coating, which comprises alkaline metal-containing silicates, can advantageously catalyze solid state reactions with soot particles. The coating of the particle filter has a silicate structure, for example, in which finely distributed alkaline metals, especially potassium, are incorporated as active catalytic coating components. The coating of the particle filters can be applied to different substrates, such as SIC or cordierite.
The coating of the particle filter allows for the passive regeneration of the particle filter on the basis of nitrogen dioxide (NO2), even when there are very small quantities of nitrogen dioxide and/or at already low temperatures, since the reaction of the soot or of the soot particles contained in the particle filter with nitrogen dioxide in the particle filter, the reaction being catalyzed with alkaline or alkaline metal compounds by the coating, is a solid-state reaction that is catalyzed, i.e., supported or brought about, by the coating. This reaction can take place at a particularly high reaction rate. Observed under the same temperature conditions, the reaction of the soot with nitrogen dioxide can occur with smaller quantities of nitrogen dioxide and at higher reaction rates in a particle filter with a coating containing alkaline and/or alkaline earth compounds than in a particle filter with a coating that contains precious metals. The active oxygen (O2)-based soot oxidation and/or regeneration is also catalyzed comparatively better using a coating containing alkaline and/or alkaline earth compounds and takes place even at considerably lower temperatures in particle filters with coatings of this type than in particle filters with precious metal coatings. Therefore, even when NO2 is excluded, particularly during the dispensing of the aqueous urea solution, soot can be oxidized with O2 to carbon dioxide (CO2) and water vapor (H2O) in the particle filter.
Particle filter regeneration should be understood to mean that at least some of the soot particles which are retained in the particle filter are removed from the particle filter within the framework of the regeneration. With increasing operation times and, therefore, with increasing numbers of exhaust gas soot particles being retained, increasing numbers of soot particles are being added to the particle filter. This addition is also known as particle filter loading. Within the framework of a regeneration, the load of the particle filter is at least reduced because the soot particles are oxidized. This means that the particle filter is, for example, oxidized with NO2 or burned off with the aid of O2 within the framework of the regeneration. The role of particle filter coatings is that of catalyzing the oxidation of the soot particles. Coating the particle filter with alkaline and/or alkaline-earth compounds permits an NO2-based particle filter regeneration for significantly smaller quantities of O2 and at a higher reaction rate compared to coating the particle filters with catalytic coatings containing precious metals.
It was surprisingly discovered that the particle filter coating with alkaline metal silicates catalyzes the regeneration of the particle filter particularly well with the aid of NO2, and so this type of regeneration on the basis of NO2, which is also called passive regeneration, leads to a sufficient soot combustion rate even with low initial concentrations of NO2, such as the raw NO2 emissions from the internal combustion engine, and that, advantageously, it is not necessary to carry out NO2-based regeneration continuously in particle filters with this type of coating, but rather that a regeneration which is performed intermittently is sufficient.
Because an O2-based regeneration of the particle filter takes place at significantly lower temperatures in particle filters with coatings containing alkaline and/or alkaline-earth compounds than in particle filters with precious metal coatings, the O2-based regeneration will support the NO2-based regeneration even at temperatures from approximately 300 to 350 degrees Celsius in particle filters with a coating that contains alkaline and/or alkaline-earth compounds. The O2-based soot regeneration can also partially replace the NO2-based regeneration within a temperature window of approximately 300 to 350 degrees Celsius if the NO2-based regeneration is restricted or fails completely due to low NO2 concentrations, as is the case when the total amount of NO2 present in the exhaust gas is consumed in the SCR reaction in the upstream first SCR catalytic converter, which is formed by the first catalytic converter part. Due to the fact that O2-based regeneration in particle filters with alkaline and/or alkaline-earth compound coatings can occur in a temperature range from approximately 300 to 350 degrees Celsius, the O2-based particle filter regeneration can be used without the disadvantage of undesirable temperature-related damage to the exhaust gas after-treatment elements occurring, which can happen with the high temperatures from O2-based regenerations of conventional precious metal-containing particle filters. This is significantly beneficial for the exhaust gas after-treatment device according to the invention, since SCR catalytic converters are especially temperature-sensitive, and high temperatures in the first catalytic converter part of the exhaust gas after-treatment device according to the invention can thereby be prevented during an O2-based regeneration.
As was already indicated, it has been shown to be especially beneficial when the combination catalytic converter, in particular catalytic converter part, is the first exhaust gas after-treatment element through which the exhaust gas passes downstream of the internal combustion engine. In other words, the first catalytic converter part, in particular the combination catalytic converter, is the first exhaust gas after-treatment element through which the exhaust gas from the internal combustion engine passes after the exhaust gas has exited the internal combustion engine so that, relative to the direction of exhaust gas flow from the internal combustion engine to the first SCR catalytic converter, there is no exhaust gas after-treatment element to perform after-treatment of the internal combustion engine exhaust gas between the first combination catalytic converter, in particular the first catalytic converter part, and the internal combustion engine. Excessive cooling of the combination catalytic converter can be prevented in this way.
In one embodiment of the invention, the second catalytic converter part and the third catalytic converter part form a hybrid catalytic converter, wherein the volume of the second catalytic converter part through which the exhaust gas is flowing is approximately twice as large as the volume of the third catalytic converter part. It has been surprisingly demonstrated that the volume ratio of the second and third catalytic converter parts according to this embodiment of the invention is necessary in order for the exhaust gas after-treatment unit to keep a concentration of newly formed N2O so low that future emissions requirements for secondary emissions can be held below real driving conditions.
The invention also includes a procedure for operating an exhaust gas after-treatment unit according to the invention. Advantageous embodiments of the exhaust gas after-treatment unit according to the invention should be considered advantageous embodiments of the procedure according to the invention and vice versa.
Further advantages, features and details of the invention are disclosed by the description of preferred embodiments that follows and with reference to the drawings. The features and combinations of features stated above in the description as well as the features and combinations of features stated below in the description of the figures and/or shown in the figures alone can be used not only in the specified combination in each case, but also in other combinations or in isolation without departing from the scope of the invention.